Contents

Abstract

Specific features of excitation of
dayside long-period
geomagnetic pulsations during the magnetic storm of 21 February
1994, under extremely high interplanetary magnetic field (IMF) and
solar wind parameters and their large variations are discussed. Two
intervals have been analyzed. The first is 1300-1400 UT under a
very high (up to ~100 nPa) dynamic solar wind pressure, and the
second is 1500-1600 UT under a high magnetic pressure and lower (to
~10 nPa) dynamic solar wind pressure. Two bands, a
low-frequency band at
f < 2 mHz and a high-frequency band at
3-5 mHz, were chosen according to the geomagnetic pulsation spectrum.
During the first and second intervals, maximum amplitudes of
low-frequency oscillations were observed at the dayside polar cusp
latitudes and in the closed magnetosphere, respectively. The
morphological characteristics of the high-frequency pulsations
(3-5 mHz) were similar during both intervals and were consistent
with the resonance nature of their generation. However, contrary to
the "classical" Pc5, the highest pulsation intensity was observed
in the afternoon
sector rather than in the morning one.
Simultaneously, similar variations of the IMF parameters were
observed. For the conditions of a high dynamic solar wind pressure
(1300-1400 UT) the dynamic pulsation spectra proved to be similar
in the interplanetary space and at the Earth's surface at the
dayside cusp latitudes. Variations in the solar wind density and
magnetic field were found to be in antiphase, which can be the
evidence of the compressional wave approach to the Earth.

Introduction

During strong geomagnetic disturbances, typical auroral zone
phenomena are detected at middle and even equatorial latitudes. For
instance, during strong magnetic storms long-period geomagnetic Pc5
pulsations (1.5-6.0 mHz),
whose maximum amplitudes under
moderately disturbed conditions are observed at
F 68o-72o,
occur at much lower latitudes.
Bol'shakova et al. [1994]
showed that a drastic compression of the
magnetosphere by the solar wind during the great magnetic storm of
13 March 1989, resulted in the excitation of Pc5
at the latitudes
53o-57o which
are not typical for this class of pulsations.
The maximum frequency in the pulsation spectra increased with
decreasing latitude from
f = 3.6 mHz at
F 57o
to
f = 4.9 mHz at
F 53o,
which is consistent with the hypothesis of
the resonance nature of oscillations
[Samson et al.,1992;
Walker et al.,1992].

Schott et al. [1998]
and
Kleimenova et al. [1998]
analyzed geomagnetic Pc5
pulsations during another, also very
strong, magnetic storm of 24 March 1991. In the daytime, two types
of long-period geomagnetic pulsations were detected at middle
latitudes. They were (1) quasimonochromatic
~1.5-2.0 mHz
oscillations with a very low azimuthal wave number ( m 1 ) and synchronous
wave packets on the global scale and (2) broadband
oscillations
in the frequency band ~2.5-3.5 mHz with
m 3-5 and wave
packets incoherent in space.

During the great magnetic storm of 5 August 1972, intense
geomagnetic Pc5 pulsations in the magnetosphere were observed at
L 4.5 [Engebretson et al.,1983].
During the magnetic
storm of 10-11 January 1997,
Villante et al. [1998]
observed a simultaneous occurrence of the 0.8-5.0 mHz ULF pulsations
at low latitudes in Europe (observatory L'Aquila,
F = 36.2 o N) and
at the antipodal meridian in the southern polar cusp (observatory Terra
Nova Bay,
F = 80.5 o S).
The authors interpreted the observed
effect as a generation of global magnetospheric compressional waves
due to a strong solar wind pressure pulse.

The 21 February 1994 magnetic storm (with SC at 0901 UT) was
caused by the interplanetary magnetic cloud approaching the Earth
[Araki et al.,1995;
Petrinec et al.,1995]. The
storm expansion phase began at ~1300 UT. It was characterized
by strong geomagnetic disturbances with
Kp up to
7+ and
AL up to
~1400 nT, the ring current intensity being relatively low
( Dst (-145) nT).

Figure 1

During the expansion phase of this storm the IMP 8
and Geotail
satellites were in the radial direction at a distance of
55 RE from each other
(IMP 8 at
+25 RE and Geotail at
- 30 RE ).
In
spite of this, both satellites detected similar, unusually large,
variations in the IMF and solar wind parameters
[Yamauchi et al.,1996],
which means that a large-scale solar wind structure
(an interplanetary magnetic cloud caused by coronal mass ejection
(CME)) approached the Earth. In the conditions of a sufficiently
high and slightly varying solar wind velocity both satellites
detected very high solar wind densities, almost up to 80 cm
-3.
The dynamic solar wind pressure was as high as ~100 nPa. The
strongest variations in the IMF parameters were observed at
1300-1700 UT. At 1330-1400 UT the IMF
Bz sharply changed from
-35 nT to
+55 nT, and
By varied from
+44 nT to
-65 nT. Figure 1 shows
the IMF and solar wind data ( B,
Bx,
By,
Bz,
n, and
v )
obtained by
Geotail on 21 February 1994. Note that Geotail was
at
X - 30 RE,
Y + 60 RE, and
Z 2 RE,
which means that the time delay of several minutes
(~7-9)
should be taken into account when the satellite and
ground-based data are compared.

The magnetic storm of 21 February 1994 was of particular interest
for the analysis because during this period not only very high
values of all solar wind parameters but also their unusually strong
variations took place near the Earth's orbit. During the interval
~1300-1340 UT the IMF variations were observed on the
background of a very high dynamic solar wind pressure
( P 100 nPa),
and at ~1500-1600 UT, they were detected under a
considerably lower pressure ( P 10 nPa).
Yamauchi et al. [1996]
showed that during this storm the first situation
(a high dynamic pressure) resulted in the expansion of the dayside
polar cusp, and the second situation (a high magnetic pressure) led
to its narrowing. Since the intervals mentioned above were
characterized by a high (up to ~750 km s
-1 )
and relatively stable
solar wind velocity, variations in the dynamic pressure actually
manifested variations in the solar wind density.

The goal of this work was to analyze in detail the ground-based
long-period geomagnetic Pc5 pulsations (1-6 mHz) in the dayside
magnetosphere during the periods of a high dynamic solar wind
pressure and large variations in all IMF components (1300-1400 UT)
and relatively lower dynamic pressure (1500-1600 UT) which was
also accompanied by high values of the IMF parameters and their
variations.

Data

Figure 2

For the analysis we used the 1-min sampling data obtained
from
the global network of ground-based stations in the Northern
Hemisphere (INTERMAGNET) and from three French observatories in the
Southern Hemisphere (AMS, CZT, PAF), as well as 10-s sampling
data of the Scandinavian network of stations (IMAGE). Figure 2
shows schematically the location of the main observatories used,
and Table 1
lists in alphabetical order their corrected
geomagnetic coordinates and codes.

Owing to the longitudinal arrangement of the selected
observatories, characteristics of the geomagnetic pulsations could
be studied simultaneously in the morning, afternoon, and evening
sectors of the Earth. For instance, during the interval 1300-1400
UT observatories NAQ ( F = 68.1o ) and STJ ( F = 55.5o )
were near
the geomagnetic noon; observatories PBQ ( F = 66.2o ) and OTT
(F =59.3o) were in the morning
sector, 0800-0900 LT; observatories
FCC
(F= 69.8o) and GLN
(F = 60.4o) were also in the
morning
sector, but in the earlier one (0600-0700 LT); and the European
and Scandinavian stations were in the evening sector. The
geomagnetic latitudes of these observatories allowed us to study
the geomagnetic pulsations in both the closed magnetosphere and the
dayside polar cusp.

To analyze morphological characteristics of the long-period
geomagnetic pulsations, the initial data were preliminary, digitally
filtered in the 0.6-10 mHz band (Pc5), and then amplitude and
dynamic spectra of the oscillations were calculated for the chosen
time intervals. After this the waves were analyzed in a narrower
frequency band corresponding to the maxima in the pulsation
spectra.

Analysis of Observations

Let us consider the morphological features of the dayside
geomagnetic pulsations detected at the Earth's surface during two
chosen time intervals: (a) 1300-1400 UT under a very high dynamic
solar wind pressure and (b) 1500-1600 UT under strong IMF
variations on the background of a lower dynamic solar wind
pressure.

Interval 1300-1400 UT

Figure 3

A spectral analysis of the dayside geomagnetic pulsations
has
revealed an increased oscillation intensity in the closed
magnetosphere in two frequency ranges: low frequencies,
f < 2 mHz,
and high frequencies,
f 3-5 mHz. Figure 3a demonstrates, as
an example, the amplitude pulsation spectra for several
observatories at 1300-1400 UT. It can be seen from Figure 3a that
the spectral distributions of the pulsation intensity in the
morning (GLN, FRD) and evening (NUR, PAF) sectors are similar, the
pulsation amplitude in the afternoon hours being nearly twice as
high as in the forenoon hours. Note the presence of well-defined
discrete peaks in the oscillation spectra at similar frequencies in
the morning and evening sectors.

Figure 4

The growth of the Pc5 pulsation amplitudes to the East and
West
from the noon meridian is also clearly seen in Figure 4, which shows
oscillations filtered in the 3-5 mHz band for four meridians: dawn
(FCC and GLN), morning (PBQ and OTT), near noon (NAQ and STJ), and
evening (SOD, OUJ, and NUR). It is obvious from Figure 4 that at
higher latitudes, at observatories FCC, PBQ, and NAQ, oscillations had a
continuous quasi-noise character, while at the remaining
observatories, the pulsations were in the form of individual wave
packets coinciding in time. It can be supposed that FCC, PBQ, and
NAQ were in the region of the
footprint
of the dayside polar cusp
and entry layers, and the other stations were in the closed
magnetosphere.

The geomagnetic pulsation amplitudes at similar latitudes
near noon
were much smaller than in the morning and evening sectors, the
amplitude in the evening sector being larger than in the morning
(Figure 4).
For instance, at 1300-1400 UT the 3-5 mHz subauroral
pulsation amplitude at observatory NUR (1700 LT) was nearly 4 times as
large as that at observatory STJ (1200 LT) located at the same latitude.
At the European middle-latitude observatories the Pc5 pulsation
amplitudes increased from the western
to the eastern
observatories.
For instance, in the longitudinal interval ~80o-115o,
at a latitude of ~50o, the pulsation amplitude at MOS was
nearly twice as high as at ESK, though ESK is located at a somewhat
higher latitude.

Wave packets on the morning and evening sides were similar, with
opposite directions of the wave polarization vector rotation. At
~1330 UT the polarization vector of the 3-5 mHz pulsations
rotated counterclockwise in the morning (OTT) and clockwise in the
afternoon sector (for instance, NUR, ESK, CLF, and others). These
facts indicate that waves propagated westward and eastward from the
noon meridian.
The phase
(j1) of the
X component oscillations at STJ
(l =31 o) was
ahead of the wave phase
(j2) at NUR
(l =103 o) by ~2 min, which
corresponds to
very low azimuthal wave numbers
[m=(j2-j1)/Dl ]
m 1-2.
The same values of
m were obtained from comparison of phase differences at observatories HAD
( 1300 LT) and AMS (~1800 LT) at
F 50o.
Note
that 1-min sampling data were used in the analysis, which means
that the time of the phase lag could be determined with an accuracy
not better than 1 min.

The reversal of the polarization vector rotation sense took place
not only in the longitudinal but also in the latitudinal
direction. At the Scandinavian chain of stations the maximum
amplitudes of the 3-5-mHz pulsations at 1330-1400 UT were
observed at
F 62o
(OUJ). On both sides of the amplitude
maximum the polarization vector rotation directions were opposite
(counterclockwise poleward and clockwise equatorward), which agrees
with the theory of field line resonances (FLR).

Comparison of observations at the quasi-conjugate observatories CLF and HER
at
F 43o
has shown that oscillations in the
X component
occur in phase and oscillations in the
Y component are in
antiphase. The same picture was obtained for observatories MOS-CZT at
F 51o
and for NUR-PAF at
F 57o.
This means that
the motion of field lines at their north and south ends was
symmetric with respect to the equatorial plane, which corresponds
to the first (odd) harmonic of the standing wave. The polarization
vector rotated in the Northern and Southern Hemispheres (CLF and
HER) in opposite directions, as at MOS-CZT and NUR-PAF.

It is evident from comparison of the spectral distributions of
pulsations at observatory BJN ( F 72o )
and observatory SOR
(F 67.5o)
that the oscillation spectra extended to higher
frequencies with increasing
latitude.
Below
F 62o
the pulsation amplitudes at
f < 2 mHz became
comparable with the wave amplitudes in the 3-5 mHz range
(Figure 5b).

Two well-defined temporal wave packets, at 1320-1345 UT and
1355-1410 UT (Figures 4-6), were observed at all observatories
located inside the closed magnetosphere. The first wave packet
contained only low-frequency pulsations with
f < 2 mHz, and the
second packet contained high-frequency pulsations with
f > 3 mHz.
This picture was especially pronounced in the morning sector at
observatories OTT and FRD (Figure 5a), and also in the afternoon time at
observatories HAN and NUR (Figure 6b) where pulsation amplitudes were nearly
an order of magnitude larger (see the scale of the graphs). In the
noon time (observatory STJ in Figure 5a) the low-frequency
oscillations
had a much greater intensity than the high-frequency
oscillations.

At the Scandinavian meridian (Figure 6b), at 1320-1410 UT, two
wave packets are also clearly seen. The first was characterized by
a strong burst of low-frequency
pulsations at polar latitudes
(Figure 6a,
NAL-BJN). Their amplitudes decreased rapidly with
decreasing latitude. Note that the bursts of the 3-5-mHz
oscillations were observed in these two wave packets both at the
latitudes of the dayside polar cusp (NAL, LYR, HOR) and in the
closed magnetosphere (Figure 6b). At lower latitudes (HAN, NUR) the
amplitudes of the 3-5 mHz oscillations were larger in the second
burst than in the first burst. A similar picture was observed in the
morning sector (FRD, OTT). The SOR and MAS stations were probably
located near the boundary between the closed and the open
magnetosphere.

Figure 7

An abrupt suppression of the 3-5 mHz oscillations (Figures 4-6)
occurred at 1404 UT and coincided with the occurrence of a negative
magnetic impulse which was most pronounced at the magnetograms of
the European observatories (Figure 7) and also with a sharp
decrease in the dynamic solar wind pressure from
P 70 nPa to
P 10 nPa
[Yamauchi et al.,1996].
According to the
Geotail data, the solar wind density decreased at this moment from
n 66 cm
-3 to
n 12 cm
-3,
the solar wind velocity
remaining nearly stable ( V 750 km s
-1 ).
The
amplitude
dynamic pulsation
spectra show that a drastic suppression of oscillations at
~1400 UT is more typical of the Pc5 pulsations (3-6 mHz). At the
polar cusp latitudes, at observatories NAL-HOR (Figure 6a), intense
( f < 2 mHz)
pulsations
also disappeared at ~1400 UT; and less
intense oscillations ( f 3-5 mHz) also sharply decayed at
~1415 UT. Similar to lower latitudes (observatories HAN-NUR), weak waves with
f < 2 mHz were then observed at higher latitudes during the entire
period studied (till ~1440 UT).

At observatories NAQ (noon) and PBQ (morning), which is located at a
close latitude but
40o to the West, the low-frequency
0.6-2.0-mHz wave packets did not coincide, though the spectra
exhibited a broad maximum in this frequency range. The pulsation
amplitude at NAQ was as high as 100 nT, and at STJ (the same
meridian but
12o lower in the latitude), it was only ~20 nT.
This means that the low-frequency oscillation amplitude decreased
sharply with latitude, as at the Scandinavian meridian. The dynamic
spectra shown in Figure 5a suggest that PBQ and NAQ were located
near the projection of the equatorial boundary of the dayside polar
cusp where intense
pulsations at
f < 2 mHz,
typical of these
latitudes, were excited. At lower latitudes (OTT and STJ), in the
closed magnetosphere, the amplitudes of these pulsations were 4-5
times smaller.

The differences between the morphological characteristics and
the dynamics of the pulsations at the frequencies above and below 3 mHz
suggest that these pulsations have different origins.

Interval 1500-1600 UT

The beginning of this interval was characterized by a decrease in
the dynamic solar wind pressure to
P 10 nPa on the background
of a very high magnetic pressure. The IMF B was greater than 60 nT
under large positive
Bz (to
+20 nT) and very large negative
By (to
-60 nT); the solar wind velocity was about 600 km s
-1 (Figure 1).

At ~1500 UT a small jump in the dynamic pressure of the solar
wind from ~4.5 nPa to ~11 nPa due to a sharp increase in
its velocity from ~550 to ~700 km s
-1 was observed. It was
accompanied by a new burst of long-period geomagnetic pulsations
with the highest intensity in the
afternoon sector, as during the interval discussed earlier.
Compared to 1300-1400 UT, only a slight enhancement of the
f < 2 mHz
pulsations
and weak higher-frequency oscillations were
observed at the polar cusp latitudes (Figures 5a and 6a).

At the Scandinavian meridian, i.e., at ~1700 LT (Figure 6a),
two bands in the wave spectrum, at
f < 2 mHz and
f > 2 mHz,
were well pronounced at the latitudes lower than
~65o,
similar to 1300-1400 UT. The low-frequency maximum had a much
higher intensity than the high-frequency one. The Pc5 pulsations
had the largest amplitudes at
f 3-4 mHz. The wave intensity
maximum was observed at
F 60o-62o
(OUJ, HAN) at
1520-1530 UT, i.e., somewhat later than the low-frequency
maximum. At the midlatitude European observatories (Figure 5b) the
main maximum in the oscillation spectrum was observed at low
frequencies,
f < 2 mHz.

The onset of the burst of the 3-4-mHz oscillations coincided
with
the sign reversal of the
Bz component, from
+25 nT to
-24 nT
(Figure 1).
At that time the change of the
By from
-44 nT to
+30 nT
and
Bx from
-17 nT to
+23 nT also occurred. In the dayside sector
of the magnetosphere (Figure 5a), observatories NAQ and STJ detected only
low-frequency ( f < 2 mHz) oscillations with much smaller amplitudes
than at 1300-1400 UT.
In
before noon hours (1000 LT), the
pulsation spectra at FRD ( F 50o)
had two pronounced
frequency bands (Figure 5a),
similar to the
pre-evening
post-noon sector (the
Scandinavian meridian), but the oscillation amplitude was much
smaller than in the evening. Figure 3b
shows the amplitude
pulsation spectra at observatories GLN (0800 LT), OTT (1000 LT), and PAF
(2000 LT). It is obvious that oscillations are considerably
enhanced in the evening sector.

Thus a sudden southward reversal of the
Bz direction led to the
excitation of the 3-5 mHz geomagnetic Pc5 pulsations with the
maximum amplitude in the afternoon hours, both at 1300-1400 UT,
under the conditions of a high dynamic solar wind pressure
(P > 70 nPa), and at 1500-1600 UT, under a lower pressure
(P 5-10 nPa).
A sharp large decrease in the solar wind density resulted in an
abrupt suppression of the Pc5 generation. In the low-frequency
range ( f < 2 mHz), bursts of oscillations occurred during both
intervals. The highest pulsation amplitudes were observed at the
dayside polar cusp latitudes during the first interval and in the
evening sector inside the closed magnetosphere during the second
interval.

Discussion

Thus on 21 February 1994, at 1300-1600 UT, a passage of a
large-scale solar wind irregularity with very strong variations in
the interplanetary magnetic field and solar wind (Figure 1)
happened in the near-Earth space. It was accompanied by bursts of
long-period geomagnetic Pc5 pulsations with maximum amplitudes in
the afternoon sector.

Many authors attribute the generation of Pc5
pulsations to
development of the Kelvin-Helmholtz instability at the magnetopause
or flanks of the entry layers of the magnetosphere flowed by the
solar wind. Generally, symmetric flowing around the Earth must give
rise to surface waves symmetric in space with respect to noon.
However, the majority of researchers [e.g.,
Chisham and Orr, 1997;
Kokubun et al.,1989;
Nosé et al.,1995;
Ol',1963;
Pilipenko et al.,1997;
Yumoto et al.,1983]
convincingly showed the asymmetry of
pulsations on the morning and evening sides of the Earth. It is
probably the result of a spiral structure of the solar wind when
the IMF direction is closer to the normal to the magnetopause
boundary in the morning hours than in the afternoon. Hence the
criteria for excitation of the Kelvin-Helmholtz instability on the
morning side are satisfied more easily than on the afternoon side
[Lee and Olson, 1980].

Comparison of the ground-based and satellite observations has shown
[Kokubun et al.,1989;
Yumoto et al.,1983] that
in the morning magnetosphere, transverse azimuthally polarized waves
well correlating with the oscillations at the Earth's surface are
mostly detected, while in the afternoon sector, radially polarized
compressional waves poorly correlating with the ground-based
observations dominate.

In the case we discuss here the largest amplitudes of the Pc5
pulsations (3-5 mHz) were observed in the afternoon
sector
rather
than in the morning
sector.
Hence it is unlikely that their source is
the Kelvin-Helmholtz instability.

The morphological characteristics of the 3-5-mHz geomagnetic
pulsations observed on 21 February 1994, such as opposite
polarizations in the morning and afternoon sectors, polarization
reversal along latitude near the wave amplitude maximum, a discrete
spectrum, the phase relations between oscillations, opposite
directions of the polarization vector rotation in conjugate
regions, low azimuthal wave numbers ( m 1-2 ), and the temporal
structure of the pulsations in the form of individual wave packets.
These characteristics
suggest that the pulsations have a resonance nature.

Besides the Kelvin-Helmholtz
instability,
the Alfvén resonance of field lines (FLR)
can also be caused, for instance, by a magnetic
impulse in the
IMF.
As a rule, FLRs are observed mainly in
the morning sector of the magnetosphere. However, some authors
reported on the cases of simultaneous occurrence of the geomagnetic
Pc5 pulsations on the morning and afternoon sides of the Earth. For
instance, in the initial phase of the great magnetic storm of 24 March
1991, quasi-sinusoidal oscillations with a period of 8-10 min,
synchronous wave packets, and maximum amplitude in the afternoon
sector were observed on the dayside of the magnetosphere
[Araki et al.,1995;
Kleimenova et al.,1998;
Schott et al.,1998].

Shimazu et al. [1995]
also described the events of
simultaneous occurrence of the geomagnetic Pc5 pulsations with
similar waveforms in the morning (0400 LT, observatory College) and
evening (1800 LT, observatory Kiruna) sectors. The
H -component variations
at observatories Kiruna and College were in antiphase and corresponded
to opposite directions of the polarization vector rotation in the
morning and evening times, as in our case. The intensification of
pulsations was observed under a high (~620 km s
-1 )
solar wind
velocity and increased dynamic solar wind pressure.
Shimazu et al. [1995]
concluded that the source of global Pc5 is
associated with the magnetosphere cavity resonance excited by the
passage of a high-pressure front of the solar wind irregularity in
the interplanetary space.

The global geomagnetic 3-5-mHz pulsations that we observed are likely
to have the same origin, namely, a global compressional wave in the
magnetosphere. This frequency range agrees with the numerical
calculations of
Kivelson et al. [1984].
The oscillations
are enhanced at the frequencies at which the wave cavity mode
matches local FLRs
[Kivelson and Southwood, 1986].
Yeoman et al. [1990]
showed that the closest interaction between
the compressional mode and Alfvén FLR
is observed for the
waves with low wave numbers ( m 1-3 ). We observed the same
values of
m for the 3-5 mHz range on 21 February 1994.

Figure 8

Potemra et al. [1989] showed
that periodical variations in
the solar wind density lead to excitation of waves propagating
from the dayside
toward the magnetotail. In turn, these waves give rise to local
Alfvén field line resonances. Figure 8 presents
the
amplitude
dynamic
spectra of variations in the IMF
B and solar wind density observed
by Geotail. Note that in order to refer the
satellite
observations to
the Earth's orbit, it is necessary to take into account the
~7-8 min lag for 1300-1400 UT and a somewhat smaller lag
( 4-6 min) for 1500-1600 UT because of the
satellite position
during the time interval analyzed
[Yamauchi et al.,1996].

Comparison of Figure 8 with Figures 5 and 6 reveals that the burst
of geomagnetic pulsations at 1300-1400 UT coincides with a similar
burst of the interplanetary magnetic field and solar wind density
variations. As at the ground-based observatories, the main
maximum in the spectrum of variations in the solar wind parameters
is observed at
f < 2 mHz.
Hence it
allows us to propose hypotheses
that the
geomagnetic pulsations can be caused by corresponding oscillations
in the
IMF which penetrate immediately into the
dayside polar cusp region.

The Pc5 generation was abruptly suppressed when a sudden
impulse
Si occurred (Figure 7). Such a sudden disappearance of Pc5 pulsation
was also observed during the magnetic storm of 24 March 1991
[Kleimenova et al.,1998].
The effect of an abrupt
suppression of the Pc2-4 pulsations on the global scale (observatory
Petropavlovsk, Kamchatka, observatory Borok, Europe, and observatory
Soroa, Cuba)
after
Si was also noted by
Troitskaya et al. [1969]
who
attributed it to a sudden expansion of the magnetosphere. However,
in our case a sharp decay of Pc5 after ~1405 UT can be
caused by changes in the conditions in the IMF, or namely, a
sudden sharp decrease of the cone angle, because the
Bx/B ratio
suddenly decreased from ~0.45 to ~0.003 under very high
values of the IMF
B -component. This confirms our hypothesis that
the source of Pc5 at 1300-1400 UT is the
IMF variations.

Note that during the interval 1500-1600 UT, contrary to 1300-1400
UT, the geomagnetic pulsation spectra on the ground did not
coincide with the spectra of variations in the IMF parameters. At
~1530 UT the intensity of low-frequency pulsations in the solar
wind was much lower than at ~1330 UT. However, the afternoon
and evening
f < 2 mHz pulsations on the ground were much stronger
at ~1530 UT than at ~1330 UT (Figures 5 and 6). They were not
observed at the polar cusp latitudes (Figure 6a). Therefore it can
be supposed that under a high dynamic solar wind pressure the
magnetic pulsations at the Earth's surface at 1300-1400 UT had an
external origin, while under the conditions of a high magnetic
pressure at 1500-1600 UT, the source of oscillations was located
inside the magnetosphere.

Figure 9

The 3-5-mHz pulsations were observed not only on the ground
but
also in the interplanetary space. Figure 9 shows variations in
B,
Bx,
By,
Bz, and solar wind density ( n ) filtered in this band. It is
seen that variations in the density and magnetic field
(IMF
Bx ) are in
antiphase, which corresponds to the compressional wave in the solar
wind.

For the interval 1330-1340 UT the Geotail measurements were not
available, and it was impossible to define whether the burst of the
ground (3-5 mHz) Pc5 at that time coincided with the onset of
pulsations in the solar wind density in the same frequency range. A
sudden suppression of the middle-latitude Pc5 coincided with a
sharp decrease in the solar wind density and disappearance of
pulsations of the same periods in the density as well as in IMF
Bz and
By. It is interesting to note that during the period when
the
first wave packet of Pc5 (1325-1350 UT) was detected, the
strongest variations occurred in the IMF
Bx (the data on density
variations were not available), which means that the waves had a
compressional
structure. During the second Pc5 wave packet (1355-1406 UT)
the strongest variations were detected in the IMF
By, though
pulsations of the IMF
Bz were also observed. Therefore the wave in
the solar wind had both the transverse and the compressional field
components,
and
source of ground Pc5
could be associated with compressional waves of
the same periods in the solar wind.
A similar excitation
of FLR due to quasi-periodical oscillations in the solar wind
parameters was reported by
Prikryl et al. [1998].

The regression analysis of the relationship between the Pc5
amplitude on the ground and the IMF parameters has shown that
during the
first interval, under the strong solar wind dynamic
pressure and
Bz>0,
Bx<0, and
By >0, the dayside pulsation
amplitude was mostly
controlled by the IMF
By (with the regression coefficients up to 0.93),
as well as it is
typical for field-aligned currents.
During the second interval, under the strong IMF
magnetic pressure and
Bz <0,
Bx >0, and
By >0,
the most effective IMF component was
Bx (with the regression coefficients up to 0.89).
During both intervals, the strongest
bursts of pulsation were observed under
By>0.

Conclusions

Thus under the extreme conditions of the magnetic storm of
21 February
1994, characterized by very high
values of the interplanetary magnetic field
and solar wind parameters and their strong variations (Figure 1),
the dayside geomagnetic pulsation spectra showed two well-defined
bands of enhanced oscillations (low frequency,
f < 2 mHz, and
high frequency, 3-5 mHz).

During the first interval (1300-1400 UT), variations in the
IMF
occurred on the background of a very high solar wind dynamic
pressure (to ~100 nPa). According to the Geotail data,
oscillations of the interplanetary magnetic field and density were
in antiphase, which can be the evidence of the approach to the
Earth of an interplanetary compressional wave causing generation of
geomagnetic pulsations at
f < 2 mHz with the largest amplitudes
near the dayside polar cusp (Figures 5a, 6a, and 8).

During the second interval (1500-1600 UT) the solar wind dynamic
pressure decreased to ~10 nPa, and no intense pulsations at
f <2 mHz were observed either in the solar wind or in the polar cusp.
However, a more intense burst of oscillations at
f < 1.5 mHz than
at 1300-1400 UT was observed in the afternoon closed
magnetosphere. The onset of these oscillations coincided with the
approach of a sharp gradient of the dynamic solar wind pressure to
the Earth. It can be supposed that in the first case the
low-frequency oscillations had the external origin, and in the
second case, the source of waves was located inside the
magnetosphere.

The high-frequency Pc5 (3-5 mHz) geomagnetic pulsations in the
dayside magnetosphere were observed during both intervals. Their
morphological characteristics corresponded to FLR (polarization
reversal near the noon longitude and in the latitude region of the
wave amplitude maximum, a discrete spectrum with coinciding maxima
in the morning and evening hours, the antiphase
H components in
conjugate regions, very low azimuthal wave numbers, the temporal
structure of the pulsations in the form of individual wave
packets). However, contrary to typical Pc5, these pulsations were
more intense on the evening side than on the morning side
(Figure 4);
the onset of their generation coincided with a sudden change of
the IMF
Bz from positive to negative; the abrupt suppression of
pulsations coincided with a sharp drop in the solar wind density.
The observation of simultaneous similar pulsations in the IMF
(Figure 9)
suggests that a possible source of FLR are IMF oscillations
in the solar wind.

Acknowledgments

The work was supported by the Russian Foundation
for Basic Research (project 98-05-64776). The AMS, PAF, and CZT
geomagnetic observatories are operated by École
et Observatoire des
Sciences de la Terre (E.O.S.T., Strasbourg, France) with logistical
and financial support of Institute Francais pour la Recherche et la
Technologie Polaires (I.F.R.T.P.). A part of this work performed in
France was supported by Centre National de la Recherche
Scientifique (project de recherche sur conventions internationales
du CNRS 2617).